As a cell balancing method for secondary batteries in the related art, a variety of methods have been proposed and commercialized, such as those utilizing resistance bypass, transformers, flying capacitors, charging power supplies, and so forth. All of these methods equalize the charge states of each of cells in a battery pack in which a plurality of cells are connected in series.
Although the cell balancing methods are generally operated so as to equalize each cell voltage, during the equalization control, there is a possibility that a balance error may be caused due to a voltage drop caused by a charging and discharging current and an internal resistance. When charging or discharging is performed with high current, this error becomes significant, and therefore, it has been conventionally proposed to conduct equalization control when charging and discharging is stopped or when the charging and discharging current is low.    Patent Citation 1:    The Publication of Japanese Patent No. 3229696